I know hat you mean, sometimes I wonder that myself but there's definitely reason for it. I'm sure there's plent of other reasons, but I can tell you what I've got on my CDRWs/DVD+RWs.
The main reason I use DVD+RWs is for backup. I make regular backup images of my data drive so I have 600MB+ files piling up. I want to keep them around for as long as possible but don't need them forever. To avoid wasting space on my drives, I put them on DVD+RWs. That way, I have them for a while but don't waste a DVD+R since I don't need it forever. I used to do this with CDRWs until the images got too big.
I also use DVD+RWs for temporary storage of movies. Sometimes I'll copy a movie to DVD+RW for my mother to watch then put burn another movie for her when she's done. There's no reason to keep them since it's just so that she can see it once (sometimes not even that), like television. Same with games. I don't like to use my good discs, especially since it might take a few weeks to get through a game and I leave it in the drive the whole time, so I'll copy it to a rewritable and play off that until I'm done.
Another thing is for testing. If I am going to burn something that I'm not sure about, that I'm not certain will work, I'll test it on a CDRW/DVD+RW first.
My most important use for them is as boot disks. I've got a bunch of tools/utilities/programs/etc. that make my life easier, and I've created a system that will allow me to quickly and easily build and burn a DOS boot disk with the latest versions of all the data and applications I've selected (the DVD version of it also includes copies of my latest data and OS backups as well.) I certainly don't need to keep these forever, since the data and software will be outdated soon, so rewritables are perfect for this.
Finally there's simple storage. Sometimes I run low on hard drive space and erasable discs are a great way to make some room for a while (sometimes I have to burn a disc to make space so that I can burn a disc) or just place files on that I don't want to bother with but don't want to just delete either.
Re-writable discs are invaluable when you need them.